In true and dubious movie fashion, The Duchess transforms a serious, carefully researched biography of historical significance by best-selling British writer Amanda Foreman into a bucket of frothy banality overwhelmed by wigs, costumes, gilt-edged ceilings, sumptuous country manors and expensive period furniture as imagined by Sofia Coppola. It looks like outtakes from the nauseating bubble-gum fantasy Marie Antoinette.

The scandalous lady in the title is 18th-century socialite Georgiana Spencer Cavendish, Duchess of Devonshire, and if the movie has any longevity, it’s because she was the great-great-great-great aunt of Diana, Princess of Wales. What else of significance defines her? As played by the photogenic but vacuous Keira Knightley, Georgiana was a trendy fashion plate, gossip-column celebrity and miserable wife, married off in 1774 by her monstrous mother (a wasted Charlotte Rampling) as a virginal 16-year-old child bride to the titled William Cavendish, the Fifth Duke of Devonshire (Ralph Fiennes, who looks like he stepped out of a Gainsborough). Poor Georgianna produced two daughters but not the son her husband craved, so he cut her out of his affections early. A brutal pig and domestic tyrant with the servants, he never took to his wife in or out of bed (although there is a scene of marital rape), but instead made her the centerpiece in a threesome with his mistress, Lady Elizabeth Foster (Hayley Atwell), that lasted for 25 years. When the duke married “the other woman” after the duchess died in 1806, everyone conveniently forgot she had been the one who brought out the sexual passion in the duchess her indifferent husband ignored. Yes, she was not only adored and powerful, but a latent lesbian, too. Lions and tigers and bears, oh my!

Playing a woman who grew in stature from a spoiled child to a woman of enormous political influence, Keira Knightley demonstrates nothing beyond the emotional depth of a Cosmopolitan cover. It’s a vain and empty-headed performance, worsened by a thin, chipped little voice (echoes of Princess Di) and the kind of glossy, superficial direction (by Saul Dibb) that relies on rosy close-ups of the star posing for 8-by-10 glossies. Mention is scarcely made of the historic footnote that at the time of the American Revolution, Georgiana was so popular and politically influential that she exerted a tremendous impact on the pro-U.S. Whig Party, eclipsing her husband’s clout in Parliament. Instead, there’s too much disposable time spent on her infatuation with drinking, gambling and flirting with the future prime minister of England, Charles Grey, played like a randy fawn by the disastrously miscast Dominic Cooper, one of the students in The History Boys. The dialogue is so absurd that you wonder if Jeffrey Hatcher didn’t write it after watching too many BBC reruns of Coronation Street. Upon hearing that the distinguished politician Charles Grey has just returned from Paris, she gurgles: “No revolution yet?” Reproaching an adversary, the duke sounds alarmingly modern (“Deal? I don’t do deals!”), and when the duchess enters a room, she’s introduced to a crowd of her admirers with “what we see her wearing tonight, I look forward to seeing the rest of you wearing tomorrow!” Can anyone prove this stuff wasn’t written by Joan Rivers?

The trappings are lush, but this is not a great, or even worthwhile, movie. Eventually, as her adulterous affairs become public knowledge, the full weight of 18th-century morality crashes down on her powdered wigs (and matching hats so elaborate Hattie Carnegie would retch), and the sad fate of Princess Di’s celebrity ancestor is sealed in scandal. Opening, of course, the unavoidable knowledge that nearly two centuries later, in 1981, history will be doomed to repeat itself.